Tag: virtualenv

virtualenv is a tool for simplifying dependency management in Python applications. As the name suggests, virtualenv creates a virtual environment which makes it easy to install Python packages without needing root privileges to do so.

To use the packages installed in a virtual environment you run the activate script in the bin directory of the virtual environment. This is fine when you’re working on the command line, but you don’t want to have to remember this step when running the debug server, and it’s hard to get that to work when the site is deployed under mod_wsgi.

To make things easier you can add the appropriate directory from the virtual environment to Python’s path as part of manage.py, or the appropriate fcgi or wsgi control script.

Recently I have been giving a lot of thought to how best to deploy websites, specifically Django powered sites. In future posts I’ll describe how I use some of tools available to deploy websites, but in this post I want to set out the goals of any system that you use to deploy a website.

What do mean when we say deployment? Clearly it involves getting your code onto a production server but the process also needs to look after any dependencies of your code. Updates also sometimes require database changes and these need to be managed and deployed with the appropriate code changes. If your website is more than just a hobby then it will also usually involve some sort of high availably set up.

The first requirement is repeatability. You might be able to follow a list of ten commands without making a mistake normally, when your site is broken and you need to get a fix deployment as soon as possible following that list will suddenly become a whole lot harder. For this reason, and to avoid the temptation to cut corners when deploying a change automation of as much as possible is key.

The second requirement is scalability. As your website grows you deployment process needs to be able grow with it. As you add a new server to your cluster you don’t want to have to spend a long time updating your deployment process, nor do you want the extra server to create extra work for you to do on every deployment.

Another requirement is speed. I’m usually very skeptical of anything that claims ‘written for speed’ as a key benefit. Unless you know something is slow, it’s usually better to make something easier to maintain than quick. An automatic process will inevitably be quicker than a manual one and whether or not your deployment process results in down time for your site the upgrade process is inevitably a risk and ideally that risk window will be kept to a minimum.

Database migrations are a tricky thing to get right. A deployment system must allow developers to track changes that they make to the database, and to make it easy to ensure these changes are applied at the right moment.

In future posts I will talk about how tools such as fabric, virtualenv, pip and south can be used to meet these requirements and ensure you never have a failed deployment again.